Does Normal CK at 22 Hours Rule Out Rhabdomyolysis?
No, a normal creatine kinase (CK) level at 22 hours post-onset does NOT definitively rule out rhabdomyolysis, though it makes the diagnosis significantly less likely. CK levels typically peak between 24-120 hours after the inciting event, and at 22 hours, levels may still be rising and have not yet reached their maximum 1, 2.
Critical Timing Considerations
The 22-hour timepoint falls within the early rising phase of CK elevation, not the peak phase. Key temporal patterns include:
- Median time to peak CK is approximately 17 hours in trauma patients, but this represents a median—many patients peak later 2
- CK levels peak between 24-120 hours depending on the etiology and exercise modality, with the optimal sampling time being 24-120 hours post-event 1
- At 22 hours, CK levels are typically still rising and may not have reached maximum elevation, particularly in exertional or delayed-presentation cases 1
Clinical Decision Algorithm
If clinical suspicion remains high at 22 hours with normal CK, repeat measurement at 24 hours is mandatory:
Assess clinical context: Look for muscle pain, weakness, dark urine (tea-colored), recent trauma, extreme exertion, drug exposures (statins, cocaine), or prolonged immobilization 1
Check urinalysis immediately: Myoglobinuria (urine positive for blood without RBCs, brown/cloudy appearance) can precede CK elevation and confirms muscle breakdown 1
Obtain serum myoglobin if available: Myoglobin has an earlier peak plasma concentration than CK, making it more sensitive for early detection 3
Repeat CK at 24 hours post-onset: This captures the early peak window for most patients 2
Continue monitoring if initial CK is borderline elevated (>1000 IU/L but <5000 IU/L): Serial measurements every 12-24 hours until trending downward 1
Diagnostic Thresholds
CK levels above 1000 IU/L (approximately 5 times normal) are considered diagnostic of rhabdomyolysis 3. However, context matters:
- Severe rhabdomyolysis is defined as CK >15,000 IU/L, requiring aggressive fluid resuscitation (>6L) 1, 3
- CK >75,000 IU/L correlates with >80% incidence of acute kidney injury 3
- Even extremely high CK levels (>150,000 IU/L) can occur without AKI in young, well-hydrated patients 4
Common Pitfalls to Avoid
Do not dismiss the diagnosis based solely on a single normal CK at 22 hours when:
- Clinical presentation is highly suggestive (severe myalgias, dark urine, recent crush injury, extreme exertion) 1
- Urinalysis shows myoglobinuria, which can appear before CK peaks 1
- Patient is a "high responder" who may reach remarkably high CK levels more quickly than average 1
- Confounding factors exist including ethnicity, body composition, and individual exercise intensity, which lead to highly variable kinetics 1
Do not wait for creatinine elevation (>1.5 mg/dL) to initiate treatment—this threshold is inappropriate for rhabdomyolysis and represents delayed intervention 1. Early fluid resuscitation is critical and should begin based on clinical suspicion and CK trends, not renal dysfunction 1, 3.
Age-Specific Considerations
Older patients require heightened vigilance despite lower CK levels:
- Older patients develop AKI at significantly lower CK thresholds than younger patients 2
- Peak CK levels are lower in older patients (median 1,637 U/L vs 2,604 U/L in younger patients), yet AKI risk is higher 2
- More rigorous CK monitoring and lower intervention thresholds are warranted in elderly patients 2
Immediate Management if Suspicion Persists
Initiate crystalloid fluid resuscitation immediately if mechanism suggests potential for progressive rhabdomyolysis, even with mildly elevated or normal CK at 22 hours 1:
- Use isotonic saline (0.9% NaCl) at 250-500 mL/hour depending on severity 1, 4
- Monitor urine output hourly, targeting >200-300 mL/hour 3
- Check electrolytes, particularly potassium, as hyperkalemia can cause life-threatening arrhythmias 1, 3
- Discontinue any potentially causative medications (statins, fibrates, supplements like red yeast rice, creatine) 1